instrument for measuring the temperature of a system. Temperature measurement is important to a wide range of activities, including manufacturing, scientific research, and medical practice.
The thermometer is generally credited to the Italian mathematician-physicist Galileo Galilei. This general principle was perfected in succeeding years by experimenting with liquids such as mercury and by providing a scale to measure the expansion and contraction brought about in such liquids by rising and falling temperatures.
The first centigrade scale (made up of 100 degrees) is attributed to the Swedish astronomer Anders Celsius, who developed it in 1742. Celsius used 0° for the boiling point of water and 100° for the melting point of snow. . It was known simply as the centigrade scale until in 1948 the name was changed to the Celsius temperature scale. In 1848 the British physicist William Thomson (later Lord Kelvin) proposed a system that used the degree Celsius but was keyed to absolute zero (−273.15 °C); the unit of this scale is now known as the kelvin. The Rankine scale (see William Rankine) employs the Fahrenheit degree keyed to absolute zero (−459.67 °F).
. Gas thermometers work best at very low temperatures. Liquid thermometers were once the most common type in use. They were simple, inexpensive, long-lasting, and able to measure a wide temperature span. . In the early 21st century, mercury thermometers were supplanted by electronic digital thermometers, which were more accurate and did not contain toxic mercury. Digital thermometers use a thermistor, a resistor with a resistance that varies with temperature.